The present invention relates to a device for determining the level of the intraocular pressure of the human eye.
The intraocular pressure of a person's eye has been recognized as an important factor in the early detection of eye illnesses. Glaucoma, one such illness which occurs quite frequently in a large segment of the population, can be detected by an increase in intraocular pressure. By careful and routine screening, this condition may be recognized and treated before it progresses to theh point where blindness occurs. Since the most obvious early signal of glaucoma is an increase in pressure which is not easily detectable without proper screening, the ordinary person may not realize that there is a danger of this illness before other serious symptoms set in and additional damage is done to the eye. This damage can be prevented by early detection of the condition. Therefore, what is needed is a means for the physician in general practice to screen his patients on a routine basis for an increase in intraocular pressure above the accepted normal pressure which often indicates the initiation of glaucoma.
Unfortunately, the screening instruments presently available to the medical practitioner are of undue complexity, requiring continual recalibration or are so imprecise that they cannot provide a reliable check for the detection of imminent glaucoma.
Intraocular pressure is determined by two basic techniques: indentation or applanation tonometry. In indentation technique, a foot plate with a weight-loaded piston, whose specific weight can be varied, is lowered until it indents the cornea and the foot plate comes to rest on the cornea. The piston is forced upward by the resistance of the cornea being indented. As the piston is forced upward, this displacement is transmitted to a lever which actuates a pointer. The pointer then reads out on an attached scale, in mm of Hg, the resistance pressure, or distensibility of the eyeball, which is taken to be the intraocular pressure. A representative instrument of this type is the well known Schiotz tonometer.
The disadvantages of an instrument of this type are the many sources of error inherent in the instrument itself, i.e., it is affected by temperature, fulcrum friction, accumulation of debris on moving parts causing increased friction and must be frequently recalibrated. In addition, it is clumsy and difficult to use and inadvertent corneal abrasion may subsequently occur. Indentation produces relatively large volume displacement in the eye, which enhances the effects of varying scleral rigidity. All of the above tends to produce erroneous readings.
In applanation tonometry, the corneal curvature is flattened or applaned by a flat piston. The piston may be of known weight and the area of applanation determined by an indirect method or it may be of known surface area and the intraocular pressure necessary to applane that area is determined by calculating the force necessary to applane that area. Devices using pistons of known surface area are (1) Mackay-Marg which uses electronic means for readout, (2) the Goldmann slit lamp which uses an optical means for readout, and (3) the Tonour which uses a pumped pressure readout.
The well know Posner tonometer uses a piston of known weight which carries a stain on its contact surface. A print of the stain remaining on the piston after contact is transferred to a paper for measurement.
The disadvantages of the Posner instrument are the indirect readout of the area applaned by a known weight and the significant volume displacement caused by the large piston weight, which may produce erroneous readings. The transfer of area of stain remaining on the foot plate, after corneal contact, to a paper strip which is then measured on a scale is very difficult to use and produces many erroneous readings and thus, does not produce a reliable screening. The Mackay-Marg, Goldmann, and Tonour are complicated, expensive and difficult to use and thus, cannot be considered "screening" instruments.
An additional type of instrument described, for example, in U.S. Pat. No. 3,597,964, to Heine, dated Aug. 10, 1971, teaches the use of an applanation-type tonometer with a testing body of precisely determined weight to applane an area of the cornea. The area applaned can be compared to the diameter which would be applaned if the intraocular pressure were perfectly normal thereby indicating whether there is any possibility of glaucoma. This invention teaches the use of means for indicating the size of the applaned area which consists of an inner circle of a given radius and an outer surrounding area having a color different from the inner circle. These areas are formed either by inserting the testing body into a casing having the separate optical elements inside or by including a pair of transparent plane parallel oppositely inclined light-refracting plates in a hollow testing body. These plates are separated by a plane which contains the axis of the testing body and have a thickness and inclination to divide the viewed field into two halves. If the intraocular pressure deviates above or below a given value, these plates assume a shape which indicates the deviation from a predetermined intraocular pressure.
In addition, Heine teaches that the circular area applaned will equal the diameter of the circular area defined on the testing body when the intraocular pressure is 28.5 gm/cm.sup.2 (or approximately 21 mm Hg).
The Heine patent teaches the use of a testing body in the form of a hollow tube using discs to close its ends and having a ring attached to the body to give it a precise weight. However, this is a needlessly complex and expensive way to construct a testing body since the precise weight may be given to a solid testing body when constructed of a plastic material formed by injection molding techniques well known in the art.
In addition, the intraocular pressure of 28.5 gm/cm.sup.2 is lower than the generally recognized critical value at which the eye should be given further testing. Studies have shown that 97% of the population have an intraocular pressure of 22 mm Hg or less in healthy eyes and that 65% of known cases of glaucoma have an intraocular pressure of 22 mm Hg or more. By choosing the value of 28.5 gm/cm.sup.2, Heine will have a larger number of patients with false positive readings who must be tested further. The value of a screening device such as this, lies in its ability to detect quickly, simply and accurately the small percentage of people with dangerous intraocular pressure. By setting the screening value too low, the accuracy of a device in detecting glaucoma is compromised.
The magnifying lens used by Heine requires that the viewing of a patient's eyes be done from directly overhead. In screening a great number of patients which this invention is designed to do, such a position can be extremely tiring to the person performing the screening. This may result in reduced efficiency of examination and an attendent increase in false readings.
Further, Heine's illuminating means are located above the piston and within the line of sight from the magnifying lens to the weighted body, thereby disturbing the line of clear vision to the applaned area. Since the lighting is provided from above to the top surface of the testing body, there is also the problem of reflection from that surface and a limitation on the accuracy with which the lower surface area can be viewed.
It is apparent that this and other devices are not fully suited to the purpose of providing an inexpensive, easily operated, accurate device for screening the level of intraocular pressure.